1. Field of the Invention
This invention relates to an ultrasonic motor which drives a rotor by a traveling wave generated on a stator.
2. Description of the Related Art
A known ultrasonic motor is disclosed in, for example, Japanese Laid-open Patent Publication No. 63-73887 published on Apr. 4, 1988.
FIG. 6 is a cross-sectional view of the known ultrasonic motor. FIG. 7a is a plan view showing radial and circumference movements of the known stator. FIG. 7b is a cross-sectional view showing axial movement of the known stator. Referring to these figures, three dimensional movements of the known stator will be explained.
Referring to FIG. 6, an external housing comprises a case 41 and a cover 42. A stator 43 is rigidly connected to the cover 42. A thick driving portion 51 is provided on the outer circumference of the stator 43. The driving portion 51 includes a plurality of equally pitched or spaced projections 49 which are integrally formed on the stator 43. A pair of piezoelectric elements 44 are adhered to the driving portion 51. A friction film 46, bonded to a rotor 45, is pinched or held between each of the projections 49 and the rotor 45. A spindle 47 has a guard portion 48 for supporting a disc spring 50. The disc spring 50 is pinched or held between the guard portion 48 and the rotor 45. The disc spring 50 presses the rotor 45 to the stator 43. The rotor 43 integrally rotates with the disc spring 50 and the spindle 47. When electric power is applied to the piezoelectric elements 44, a traveling wave is generated on the driving portion 51 of the stator 43 due to deformation of the piezoelectric elements 44. The projections 49 are oscillated not only along a circumference direction A but also in a radial direction B due to the traveling wave which is generated on the driving portion 51. These directions of oscillation are shown in FIG. 7a. In FIG. 7a, dotted lines show the positions of the projections 49 before the traveling wave is generated on the driving portion 51. In addition to these two directions of oscillation, the projections 49 are also oscillated in the thickness or axial direction C as shown in FIG. 7b.
Due to these three dimensional oscillations A, B, C, the driving portion 51 makes a spherical locus with respect to a point D on the stator 43. However, only two dimensional oscillations are required for generating the traveling wave. Therefore, in the conventional ultrasonic motor shown in FIG. 6, the oscillation in the thickness direction C is useless for generating the traveling wave and wastes the supplied energy.